They will kill and eat nearly anything, a lot of people with smasher type of mantis find feeding them a snail or crab actually keeps them healthier. They are a very interesting animal and worth having a species only tank IMO.

My lfs has zebras for 15 and peacocks for 30. Eventually all fish will be prey as mantis grows. So species only would be best way to protect your investment. Also, They are messy eaters and can quickly fowl water. Knowing all that, I still want one. Thought about putting it in fuge but seems counter productive.